Computers often include disk drives, such as magnetic and optical disk drives, on which data can be written and from which data can be read for later use. A primary trend in disk drives is that the areal size of a bit which may be reliably written and read continues to decrease at a rate of more than 50 percent per year. Consequently, the width of a track containing a sequence of such bits must diminish at roughly half this rate. Assuming this situation continues, an advanced magnetic disk drive, which currently has a track density of 4000 to 5000 tracks per inch (tpi), is likely to have 20 to 25 ktpi by the year 2000. As disk drives are reduced in size and their recording density is increased, the head (transducer, objective lens or mirror) positioning resolution provided by traditional single-stage actuators either has reached or is rapidly approaching a track density limit. The geometry and form factor of disk drives limit the relative lever arm length of single-stage actuators, as well as reduce the size, power and efficiency of the voice coil motor, and further track density increases are thereby limited unless alternative approaches are implemented.
The typical single-stage actuator for a disk drive includes an armature arm attached to an actuator arm, with a pivot therebetween. A head is mounted on a load beam attached to the actuator arm. The armature arm is driven, typically by a voice coil motor, about the pivot to move the head relative to the disk media. The actuator must be confined within the disk drive and be movable with respect to the disk media similarly located within the disk drive. Any increase in the length of the armature arm which might serve to increase the resolution of the positioning of the actuator arm is severely limited by the overall size limitations placed on the disk drive. Similarly, the travel of the armature arm is limited by the structure of the disk drive.
The disk media used in optical and magnetic disk drives already permit high recording density. With disk media having very fine resolution recording capability, the actuator and the actuator positioning resolution, more particularly, becomes the recording limiting factor.
Typically, optical disk drives, such as compact disk (CD) players and compact disk read-only memory (CD-ROM) drives, have a two-stage actuator that provides very fine positioning resolution. These two-stage actuators have a secondary actuator (SA) that rides piggyback on a primary actuator (PA). Such multiple-stage actuators decouple the actuator inertia and stiction to increase the electromechanical zero bandwidth. The PA handles course low bandwidth (low frequency) movements, and the SA handles fine high bandwidth (high frequency) movements to keep the track misregistration (TMR) to a minimum. These two-stage actuators require a sensor, in addition to the head, that measures the relative position between the PA and the SA. As disclosed in U.S. Pat. No. 5,060,210 issued Oct. 22, 1991 to Fennema et al., the relative position sensed by an additional sensor is supplied to the PA for causing the PA to follow the motions of the SA.
The use of such an additional sensor has many disadvantages, including cost and mass, and the need for signal cabling thereto. These disadvantages become more prominent as the number of head/disk interfaces and hence secondary actuators in the disk drive increases. For example, a drive with four disks having a total of eight disk surfaces and eight heads would require eight sensors, each sensing the relative position between a PA and one of eight secondary actuators riding piggyback on the PA. Moreover, noise from each sensor can adversely influence the read performance of the head.
Furthermore, the range of applications for which disk drives are being used has changed from one in which stable, relatively massive units are protected from substantial operating shock and vibration. Disk drives are now being used in portable or other light-weight applications, such as laptop or notebook computers, which allow the drive to be exposed to greater shock and vibration, in any orientation. The offset between the actual head position and the track center is called the track misregistration (TMR). As the track pitch decreases, the allowable level of TMR drops and any increased level of shock and vibration becomes even more troublesome.